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I have been searching the internet for possible solutions to a recent problem that has occurred at our facilities. I came across TFP and was thoroughly impressed with the knowledge and communication skills of your chemistry experts. They are way beyond my knowledge so I am hoping they will be able to help me sort out the problem that has recently occourred that I cannot isolate.

I am responsible for 20 commercial indoor pools in seven facilities and, of late, we have been having the problem of ORP not tracking on three of the pools at three different locations. The complete automation system has been changed out on each of them without solving the problem and the manufacturer of the equipment has said it is a site problem. Two of these pools are over five years old but one is just over one year. Not all of the pools at a facility are affected to the point of not controlling ORP - some of the them are just slightly erratic. The automation has software that allows me to plot the sensor readings for the last two months. The plots show pH to be tightly controlled (almost a flat line) and in agreement with hand readings. The ORP plot shows sensor readings randomly and slowly driving with no relationship to FC hand readings. It is not unusual to see FC change by 8.0 ppm with a decrease in ORP. At the moment we are just turning the SWG's on and off as necessary to maintain FC levels.

All of our pools have high bather loads and we are open 24 hrs/day.
We have been using SWG's since 1996.
We do not use CYA in any of our pools.
All of our pools are automated (pH, ORP, heater control).
We have two brands of generators.
We use sand filters.
We generate bromine in our spas with standard SWG's.
We use sulfuric acid for pool pH control and CO2 for spa pH control (which is somewhat problematic).
We use monopersulfate weekly.
We remove phosphates from our water.
We have never had an algae problem.
We only rarely find it necessary to "shock" our pools. There is normally no chlorine odor in the air or a clorine smell on skin or hair.
Water systems in the area use chloride for system sanitation. In one location, CC test reads 3.0 on fill water; one system has 180 alkalinity incoming.
We have DHU's with pool-water reheat that supply adequate fresh air to the pool areas.

We have done prliminary checks of electrial systems and bonding connections. If this was an electrical or bonding/grounding/stray current problem would it not affect the pH sensor as well?

We tend to specialize in residential pools around here, so I am not sure how throughly we can answer your question.

There are a couple of things I know of that could be causing issues. I am not sure how they apply in your situation, but perhaps they will give you some hints on things to investigate further.

First, MPS will raise ORP readings and thus tend to drive FC down in systems using ORP automation. The lifetime of MPS in the water depends on several things and can be quite short to several days. The symptom would be depressed FC readings following application of MPS, rising back to normal after minutes to days.

Second, dissolved hydrogen gas will lower ORP readings and thus will tend to drive FC up in systems using ORP automation. It can take from several hours to several days for hydrogen to come out of the water. Hydrogen gas is created by the SWG cell as part of normal operation. How much of the hydrogen dissolves in the water depends on a number of factors. In many pools it will simply bubble to the surface, while in others a substantial portion will dissolve. The symptom here would be the SWG cell activating and remaining on while ORP readings fall and FC levels rise. At some point, assuming no other factors enter in, hydrogen would become saturated and the very high FC level would finally turn the cell off. At that point, assuming low bather load, ORP readings would rise slowly even with slowly falling FC levels for many hours (or even a day or two), or until the cell was next activated.

Have you looked for common factors in the pool history of the affected pools? Did problems start when the SWG systems were installed, or can the start of the problem be matched up with any change in operations or environment? Things to watch for include the install or significant modification of the SWG system, local water company starting to use orthophosphate and/or switching from chlorine to chloramine for disinfection, any significant changes in plumbing, etc.

[EDIT]Ignore my bonding comments.[/EDIT]

By the by, I have heard that long term use of sulfuric acid can cause problems, possibly shortening the lifetime of both SWG cells and plaster.

I agree with Jason's review of your issues. Except the bonding issue. I've had several ORP issues with the Pool Pilot as a result of broken/corroded bonding wires, or the grounding rod driven into dry sand rather than "earth" to discharge stray voltage. Remember, ORP is measured in millivolts.

I would also check your ORP probe and make sure it's not coated with chemicals, oils or junk that can coat the probe. Remove and clean with a soft cloth and a degreasing dish detergent. Rinse and replace.
Speaking of probes, a GOLD tipped probe is more accurate than a PLATINUM tipped ORP probe.

pH probes are not usually as affected as the ORP probes are as it's comparing the pH of the water to a reference pH gel within the pH probe. The ORP is measuring a voltage in the water, which can be affected by many things.
Sunlight (you didn't mention if your indoor pools have any exposure to sunlight)
Oxidation (which you mentioned the use of Monopersulfate. Do you also have an ozone generator or UV system?)
Cya levels (which you do not use. However, have you ever shocked or used a granular tri-chlor or dichlor, or tablets? All of which contain cya)

ORP control systems will fluctuate 20 - 40 mv and may be considered normal, but not to fluctuate your FC readings 8 ppm.

In some cases, a Current Collector may need to be used to help discharge stray voltage.

We at AIS have been developing commercial systems for quite a while now, and did a lot of research into ORP controllers. Your problem seems to be quite common with most of ORP controllers available on a market. Most of the time they work, but in some installations they don't.

I will refer your question to Val, he is responsible for R&D at AIS, see what he has to say about it.

if you don't mind me asking, which systems(SWG and ORP) are you using?

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Active member

Hello bubblehead, guys here covered topic well, not much to add. I can suggest you to come back to ORP system manufacturers and request them to 'alter" programming for the unit as the trial. What you need to try is to sample ORP after chlorinator been off for some time:
1. Chlorinator - off/ ORP - off
2. Chlorinator - on (timer mode) / ORP - off
3. Chlorinator - off / ORP - off ( timer mode)
4. Chlorinator - off / ORP - on ( average reading - timer mode)
5. go to step 1
You would want to keep us much time gap as possible between chlorinator been stopped and sample of ORP been taken. This allows ORP reading to stabilize during this time. More time is better.
In reality this is "band aid" solution and not fixing your problem but rather trying to get around it. There is also no guaranty it will work but It have helped us to stabilize the readings in some cases.

Personally, I would disabled ORP on 'problematic" installations and used manual method unless you like to gain some experience and use the sites as examples of problematic installations and do some R&D there....

Well-known member

An easy test for stray current effect on ORP probes is to make note of ORP readings with the probes in the flow cell then remove the probes (both pH & ORP due to isolation issues on some controllers) from the flow cell and put them in the same pool water but in a paper cup. If the readings are the same, you have no stray voltage issues. If the readings change (typically ORP less in cup) then you need to use a grounding electrode...preferably from the inlet line on the flow cell to a good earth ground.

Other random thoughts...

Your readings show 760 mV. If they stray much higher into the 800's control will be difficult at best due to the typical FC/ORP curve. The mV change vs. FC change may be less than your control hysteresis.

Hydrogen gas could also be part of the problem and configuration (where on the circulation system inlet and outlet are plumbed in) of flow cell lines could effect how much hydrogen gas reaches the probe. Gold probes will likely not solve the problem. The only advantage they have is the gold is not scavenged by hydrogen like platinum is so they won't be permanently damaged like platinum. They will still read a lower or negative reading due the the hydrogen (a reducer, the "r" part of ORP). They will also read about 200mV lower in normal operating conditions than a platinum electrode and gold is more subject to salt corrosion.

Jason - would you be so kind as to elaborate on this? I've been a dilute (35%) sulfuric user for years and have never heard of plaster or salt cell issues. The only thing I've heard is the potential for sulphate buildup if enough freshwater isn't introduced.

G

Guest

The sulfates are the issue. Some salt cells rare earth coatings are prone to damage from sulfates, and sulfates are suspect in some forms of plaster damage and soft spot etching. You are correct that this only occurs if the water in the pool is not diluted but with the rise in popularity of large cartridge filters this is exactly what occurs. (Cartrdige filters are actually an advantage with SWGs since the CYA and salt will not decrease with each filter cleaning.) Sulfates can also be introduced into the water by MPS use. Main advantage to sulfuric acid is that it is non fuming. IMHO, that does not really outweigh it's potential disadvantages.
How fast this buildup of sufates can occur depends on many factors. In areas with an extended swim season, such as Florida, it is more likely to occur then in areas where a pool is closed and winterized after a 3 month swim season so all these factors need to be taken into account.

Member

Thanks for your responses. Looks like quite a few excellent suggestions and some questions for me to answer. So let's get started.

We have replaced the entire controller, wires and probes on two of the pools. The symptoms remained the same.

The equipment we are using is as follows:
Acu-Trol AK-200's, AK-600's, and AK-110's. ORP probes are all the gold variety.
Generator cells are Clormatic COM25 units with some pools using a multiple parallel configuration and, on one spa, a single COM25. Current flow in these cells about 25 DCA under normal conditions. Small pools (spas) use Clormatic 601 cells in single or multiple parallel configuration.
We have been using the Clormatic since 1996 with great success.
18 months ago we began using the ECOmatic commercial units, under private label, which have been absolutely trouble free.
We do not use UV or ozone.

As we build new facilities, SWG's are installed during construction. We wait 4 weeks after filling before adding salt and energizing the cells. We use this time to calibrate the control system. The system will monitor ORP and feed traditional chlorine without CYA. The pools we are discussing have worked well in the past and have only recently developed the tracking problems. None of them are in the same county or water system and all of the water systems have been using combined-chlorine water sanitation for some time. The only thing we have noticed is that the level of CC is going up everywhere. As I mentioned in my first post, one system is over 3.0 and that is where the worst tracking problem is. Might there be other chemicals we don't know of?

We measure the MPS level with a Taylor test kit and refer to the results as the oxidation level. When it falls to near zero we dose the pool with the mfg's recommended dosage of 1 lb/10,000 gal. This usually works out to about 3 doses in 2 weeks in a busy pool. We do not get complaints of skin irritation. We use DEOX to remove MPS oxidation from water samples when we do chlorine testing.

We also use phosphate remover regularly and do not use metal-out or stain treat.

About the hydrogen, is there a test for it. I have also heard something about aerating the water, injecting air through something like a CO2 diffuser tip I guess, just before the cell. Is this something that will eliminate or reduce the hydrogen or what would this be for? What would you expect to see on a pH or ORP plot if you were able to quickly, say over two hours, remove the hydrogen or add it. Is there anything we could do that would cause the hydrogen to gas-off?

I do know how to program and calibrate the Acu-Trol's fairly well but the general programming is pretty much the same for all of our pools. It would be possible to increase the cell off-time significantly and run plots of the ORP and pH probe readings over long periods of time. The system would record the readings at 15 min intervals for a couple of months if necessary. I'll give that a try over a week and see what that shows.

Today I ran two wires from the ground and neutral bars in the pool equipment panel (low voltage 120/208V) out to the edge of the pool. Gnd and neut are not connected together in this sub-panel but the gnd bar IS connected to the pool bonding loop. I measured voltage from both wires to the pool water and found I had 2-300 mv, both AC and DC. I was using a very good clamp-on meter that will measure DC current down to milliamps but the voltage probes on this meter are not very special. I am going to measure this voltage again tomorrow with a better meter. Any tips on the bonding/grounding/stray voltage issues regarding ORP and testing methods?

About the hydrogen, is there a test for it. I have also heard something about aerating the water, injecting air through something like a CO2 diffuser tip I guess, just before the cell. Is this something that will eliminate or reduce the hydrogen or what would this be for? What would you expect to see on a pH or ORP plot if you were able to quickly, say over two hours, remove the hydrogen or add it. Is there anything we could do that would cause the hydrogen to gas-off?

I don't know of a commercially available dissolved hydrogen gas meter. I have heard of some labs that have built them. I haven't done a really through search though.

Aeration would tend to reduce the concentration of dissolved gasses that were above their equilibrium concentration (as we postulate hydrogen must be). I have no idea how effective it would be in practice. I have heard of ultrasonic degassing approaches that should work but I don't know of anyone who has actually tried them.

Bubbling hydrogen gas through a water sample will reduce it's ORP reading dramatically. The ORP reading will return to the original value over the next 6 or so hours (for cup sized samples left exposed to air at room temperature).

The only approach to compensating for dissolved hydrogen that I know of is to drive hydrogen towards it's saturation point by running a subset of the SWG cells continuously while controlling other cells via ORP.

There is remarkably little literature on this whole subject. It isn't easy to experiment with and much of what I know is a mixture of deductive reasoning and experiments that haven't produced particuarly definitive results.

Active member

In the tests we performed with ORP sensors VS H2 generated by chlorinator there was no solution found, other than use commercial cyclones (machine that removes H2 from the water flow).
On the problematic installation we observed "unstable" ORP readout while chlorinator was operational. The reading started to stabilize after 3-4 hours of chlorinator been switched off (24 hour water circulation). However, time needed for ORP to stabilize after switching off chlorinator vary from pool to pool.

Well-known member

In the tests we performed with ORP sensors VS H2 generated by chlorinator there was no solution found, other than use commercial cyclones (machine that removes H2 from the water flow).
On the problematic installation we observed "unstable" ORP readout while chlorinator was operational. The reading started to stabilize after 3-4 hours of chlorinator been switched off (24 hour water circulation). However, time needed for ORP to stabilize after switching off chlorinator vary from pool to pool.

Active member

In the tests we performed with ORP sensors VS H2 generated by chlorinator there was no solution found, other than use commercial cyclones (machine that removes H2 from the water flow).
On the problematic installation we observed "unstable" ORP readout while chlorinator was operational. The reading started to stabilize after 3-4 hours of chlorinator been switched off (24 hour water circulation). However, time needed for ORP to stabilize after switching off chlorinator vary from pool to pool.

Member

I have an 8 day plot of ORP and pH but is too big to post here. If anybody is interested in looking at it let me know and I will send you an email.

I have hand plotted FC hand readings and calculated ORP values on the system generated plot. It clearly shows an oscillation in ORP that is not affected by large changes in FC. The pH plot is extremely steady and our hand readings support the system pH readings exactly. A pH/FC/ORP relational chart is included which is where I got my "calculated ORP". If you look at this plot remember that the automation components have all been changed.

Active member

Active member

Hello bubblehead, got your charts and had a look at them today. ORP seem to be working correctly indicating â€˜water oxidizing potentialâ€™. You can see how it rises in mid-night and drops out to mid-day (probably due to UV and bath load). So from electronics/software point of view there are no problems.
ORP is not directly linked to free chlorine and there â€˜somethingâ€ in the water shielding it or deactivating oxidants. The problem is not hardware but water chemistry. With reference of what it can be you will need to lease with Acu-Trol.
Got to remember that for example at ORP=750 you can have 1ppm OR 10ppm free chlorine. ORP is not your chlorine level but oxidation potential of the water. On some swimming pools (read as water body with given chemistry) it will fairly close to free chlorine readout, on others it will be pretty far off.
What is important is that with ORP controller fitted, does water ever goes green? If not â€“ itâ€™s doing itâ€™s job. Health departments chase â€˜free chlorineâ€ indication as point of reference and not ORP which in reality more accurate indication. But this is another story altogether.

Any other thoughts anyone?

P.S. If the drama believed to be caused by chlorinator (H2), simply turn it off for 4-5 days and dose pool manually with chlorine. This should fix the problem with ORP readout if it is the H2 what causing it

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